Nanobubbles and gas-liquid mixtures for enhanced carbon dioxide sequestration

ABSTRACT

The present invention discloses a novel process for the mineralization of CO2 in mafic and ultramafic rocks or storage of CO2 in geological formations through the generation and use of nano-sized CO2 bubbles injected into a fluid-mixture.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims benefit of priority to U.S. Provisional Application No. 63/303,060 having a filing date of Jan. 26, 2022 which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to an environmental solution relying on CO₂ storage and enhanced CO₂ mineralization in mafic and ultramafic rocks. More specifically, the present invention relates to an enhanced method for CO₂ storage and mineralization by forming nanobubbles of CO₂ gas in a fluid mixture that is injected through a borehole into a geological formation for mineralisation and/or storage.

BACKGROUND

The race to enhancing and increasing the removal of CO₂ from the environment is leading to a number of innovations in this area, from rapid removal from the atmosphere to removal from subsurface storage to removing from industrial applications. Innovation in the various means of measurement of rate and quantity of mineralized and stored CO₂ is also a related area.

The oceans take up CO₂ from the atmosphere and are responsible for absorbing around a third of the CO₂ emitted by fossil fuel burning, deforestation, and cement production since the industrial revolution (Sabine et al. 2004). While this is beneficial in terms of limiting the rise in atmospheric CO₂ concentrations and hence greenhouse warming, there are direct consequences for ocean chemistry. Ocean acidification describes the lowering of seawater pH and carbonate saturation that result from increasing atmospheric CO₂ concentrations. There are also indirect and potentially adverse biological and ecological consequences of the chemical changes taking place in the ocean now and as projected into the future (Barker et al. 2012). Enhanced CO₂ mineralization is a recognized way to store and or mineralise CO₂ underground or above ground. There is a need for an innovative process for CO₂ gas-liquid injection for underground storage and/or mineralisation which may provide for much higher mass transfer ratios and buffer capacities when compared to traditional technologies, thereby increasing dissolution rates and concentrations of CO₂ within a fluid-mixture.

The present disclosure provides a method wherein nanobubbles of CO₂ gas are injected and dissolved on-surface (e.g., above ground) in a storage tank or injection pipes. Compared to traditional sparging technologies that often result in gas bubble sizes in the micron level, this method proved to be more efficient. Micron-sized bubbles proved to be challenging to handle during on-surface sparging because of the larger size of these bubbles, there is a higher potential for coalescence of micron-sized bubbles to take place while injected into a fluid stream. Coalescence would in turn result in the formation of even increasingly larger bubble sizes. These coalesced larger bubble sizes would rise faster and quickly to the surface of the injection fluid. Once at the surface of the fluid, these bubbles would burst due to buoyancy effect, releasing the CO₂ back into atmosphere resulting in decreased efficiency of CO₂ mineralization. This is highly challenging to control when injecting large volumes of CO₂ rich fluids at greater depths due to increasing hydrostatic pressures. However, with the proposed CO₂ gas-liquid injection process for underground CO₂ storage and/or mineralization, there is a reduced effect of coalescence and buoyancy as the CO₂ is injected as nanobubbles and dissolved into a pressurized fluid-mixture stream.

The present process provides for several advantages over existing technologies since it provides higher mass transfer rate; thermodynamically metastable bubbles that can remain suspended in fluids for extended periods of time; higher buffer capacities; higher CO₂ mineralisation or storage volumes; less depth of CO₂ injection; and less fluid to be injected.

SUMMARY

One embodiment of the present disclosure is drilling vertical and/or horizontals wells into mafic or ultramafic rock formation and injecting water rich of nanobubbles of CO₂ to react with said rocks producing carbonate rocks and therefore permanently entrapping CO₂.

Another embodiment of the present disclosure is a method to decrease the amount of fluid required to achieve CO₂ mineralization. By utilizing nanobubbles, higher CO₂ concentration can be obtained in the same volume of fluid resulting in an enhanced efficiency due to increased mass transfer properties. It also means that injection depths can be shallower making it more feasible to store/mineralize CO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings show embodiments of the disclosed subject matter for the purpose illustrating the invention. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 is a process flow diagram of an embodiments of the disclosed subject matter;

FIG. 2 is a schematic representation of an embodiment of the disclosed subject matter;

FIG. 3 is a schematic representation of an embodiment of the disclosed subject matter;

FIG. 4 is a schematic representation of an embodiment of the disclosed subject matter; and

FIG. 5 is a schematic representation of an embodiment of the disclosed subject matter.

DETAILED DESCRIPTION

Objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Herein, the use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one,” but it is also consistent with the meaning of one or more,” “at least one,” and “one or more than one.” The term “about” means, in general, the stated value plus or minus 5%. The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or the alternative are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and/or”.

Referring now to FIG. 1 , the embodiments described include a process for dispersing CO₂ gas into a fluid for the purpose of forming nanobubbles. Exemplary laboratory and/or field trial scale processes can utilize a fluid mixture that is originally stored on-surface and is pumped at a pressure of about 10 bar, CO₂ injection mass flow rate of about 0.46 g/s, H₂O injection flow rate of about 0.15 l/s and an injection depth of about 110 mbgl into the injection well. Other conditions such as a pressure of 5-50 bar preferably 10-40 bar of 15-30 bar, a CO₂ injection mass flow rate of 0.05-5 g/s, preferably 0.25-2.5 g/s or 0.5-1.5 g/s, an H2O injection flow rate of 0.5-10 l/s, 1-5 l/s or about 2.5 l/s, and an injection depth of 25-1000 mbgl, 50-500 mbgl or about 100 mbgl can also be used. A pressurized CO₂ (about 50 bar preferably 5-1000 bar, 10-500 bar or 100-25 bar) is regulated through a manifold and set of mass flow controllers prior to injection into the pressurized fluid mixture stream. Calculated mass flow set points control the overall CO₂ injection process in relation to fluid mixture flows. For an example, to inject 1 metric tonne of CO₂ per day at 900 mbgl zone, a fluid mixture flow rate of 18 l/min and 40 bar pressure are preferably used. These values (e.g., pressure, CO₂ injection mass flow rate, H₂O injection flow rate, injection depth, mass flow set points, and fluid mixture flow rate of 18 are exemplary and can be scaled upwards for commercial scale process by factors of 10×, 100×, 1,000×, 10,000× and/or 100,000×.

Referring to FIG. 2 , some embodiments of the disclosed invention include a water looping system having a water storage module (210), a carbon dioxide injection module and injection well (220), a mafic or ultramafic formation, here schematically illustrated as a horizontal layer (P), and an observation well module (230) for monitoring and controlling carbonation reactions. The process begins by identifying a suitable location with the rock layer being preferably at least 0.1 km thick or at least 0.5 km thick. An injection borehole is drilled into this rock layer. Said borehole is preferably at least 0.5 km deep and up to a maximum of 1.8 km in depth with the preferred depth being between 0.8 to 1.2 km deep or about 1 km mbgl. An observation borehole is drilled alongside the injection borehole with hydraulic connection between the two holes. The injection borehole is fitted with an engineered well casing (preferably steel or concrete) which is perforated at the targeted areas for mineralization in the geological formation (see FIG. 3 . and the text further below for more details). In the continuous injection process water is first pumped from the observation borehole or another source to a buffer storage tank on surface. The buffer tank is fitted to receive water from different sources such as underground water resources, sea water, treated water etc. The water, at ambient temperature, is then pumped at pressure through the injection pipeline to the injection borehole well head by using a set of booster pumps. The formation temperature at 1 km depth is about 60-80° C. Pumping water to this formation will also help controlling the temperature at the target zone and, consequently, gives greater control over the reaction rate.

Referring to FIG. 4 , the embodiment describes a nanobubble generator. The generator comprises a pump (1), an inlet (2), an air pressure gauge (3), an air connection (4), an air flow meter (5), a pump pressure gauge (6), a discharge flow valve (7), and a starter (8). The bubble generator typically operates at flow rate of 6-45 m³/hr and a maximum liquid pressure of 1.5 bar. Typical operating conditions include: temperature is 5-60° C., preferably 30-40° C., and CO₂ pressure of 1-8.5 bar, preferably 3-4 bar.

Another embodiment of the present disclosure is a programmable logic controller that automates the CO₂ injection process by controlling the CO₂ mass flow controllers, fluid-mixture booster pumps and respective valves to enhance CO₂ mass transfer ratios into the fluid mixture stream and achieving maximum buffer capacities per unit volume.

In an embodiment of this disclosure, the proposed gas-liquid injection process of the present invention dissolves bubble sizes of 10-900 nm, 25-750 nm or 50-500 nm, preferably 60-550 nm in average diameter in fluid-mixtures. The bubbles are neutrally buoyant and can remain suspended in fluids for a period of time from 15 days up to 3 months without rising to the surface. This allows for shallower injection, higher gas transfer ratio, as well as increased CO₂ concentration in fluids. Studies have shown an enhanced mass transfer coefficients of 0.35 If′ which is 11 fold better than regular bubble sizes, e.g., bubbles having an average diameter in the micron range.

In a further preferred embodiment of the present disclosure, CO₂ gas-liquid injection relies on nanobubble membrane generators that generate billions of CO₂ bubbles. Referring to FIG. 5 , at 2 barG average count is 3.66×10{circumflex over ( )}8 bubbles per ml; at 3 barG average count was 9.1×10{circumflex over ( )} 8 bubbles per ml; and at 4 barG average count was 9.71×10{circumflex over ( )}8 bubbles per ml. A nanoparticle analyser is used to measure average bubbles number and size through Brownian motion estimations. The analyser incorporates three lasers with different wavelengths and a colour camera to visualize the displacement of bubbles from 10 nm to 15 microns. Displacement is interpreted as Brownian motion, or, for larger bubbles, settling or creaming and can therefore be readily converted to particle size for each bubble, allowing high-resolution size distribution analysis. Same technique is also used to measure average bubbles number and density for specific volume of fluid. The analyser uses a cuvette that includes a black insert which houses a magnetic stir bar to keep larger bubbles suspended and mix the bubbles between videos. Samples are continuously collected from nanobubble membrane generators and transferred directly into the system cuvette without further preparation to measure average bubbles size and density.

The nanobubbles are then injected at high pressure (>25 bar) into a fluid-mixture. The generated nanobubbles' buoyancy is insignificant allowing for extended suspension within a fluid resulting in increased mass transfer within the fluid. The nanobubbles are thermodynamically metastable allowing for high residence time in fluids up to a few months.

Mafic and ultramafic rocks can contain silicate minerals including olivine, serpentine, pyroxene and plagioclase. Olivine rocks often contain magnesium, oxygen, and silicon. Olivine is the most abundant mineral in the earth's mantle until a depth of 700 km. The composition is usually a combination of SiO₄ and Mg²⁺ and minor Ca^(2±). Typically, silicon bonds with 4 oxygen molecules forming a pyramid structure so that the charges of cations and anions are balanced, and Mg²⁺ occupies the empty space between the SiO₄ structure. These bonds can be easily triggered to react with carbonic acid. The reaction of olivine with CO₂ can be accomplished by the following reaction pathway:

MgSiO₄+2 CO₂→2 MgCO₃+SiO₂  [1]

It is also proven that the rate of reaction increases significantly by introducing water. Water helps CO₂ to be solubilized forming carbonic acid and therefore making the mineralization and ion exchange process far easier and more efficient. Below is the reaction pathway in presence of water:

CO₂+H₂O→H₂CO₃→H⁺+HCO₃ ⁻  [2]

Mg₂SiO₄+4 H⁺→Mg₂++SiO₂+2 H₂O  [3]

Mg²⁺+HCO₃ ⁻→MgCO₃+H⁺  [4]

Some mafic and ultramafic rocks contain mainly the mineral's olivine and pyroxene. In the presence of water and CO₂, the following reaction occurs:

4Mg₂SiO₄(olivine)+CaMgSi₂O₆(pyroxene)+CO₂+7H₂O→3Mg₃Si₂O₅(OH)₄(serpentine)+CaCO₃(calcite)  [5]

Another aspect concerns a method for carbon dioxide sequestration utilizing pyroxene minerals. Pyroxene is one of the groups in an inosilicate mineral, which is also abundantly found out in mafic and ultramafic rocks. The general chemical formula for pyroxene is AB(Si)₂O₆, in which A can be one of the ions like magnesium, aluminum, etc. Most commonly, pyroxene can often be found as Mg₂SiO₄ and CaMgSi₂O₆. Naturally, pyroxenes react with CO₂ according to the following equations:

Mg₂SiO₄+2 CO₂→2 MgCO₃+SiO₂  [6]

CaMgSi₂O₆+2CO₂→CaMg(CO₃)₂+2SiO₂  [7]

However, similar to olivine, water increases the rate of reaction, therefore, in presence of water, below is the reaction pathway for CO₂-pyroxenes reaction:

4Mg₂SiO₄+CaMgSi₂O₆+CO₂+6H₂O→3Mg₃Si₂O₅(OH)₄+CaCO₃  [8]

CaAl2Si2O8+CO2+2H2O→CaCO3+Al2Si2O5(OH)4  [9]

The present disclosure relates to a method that utilizes the above reaction pathways (especially equations 2-9) to convert and/or store CO₂ into mafic and ultramafic rocks, as defined above, as a first aspect of the invention. The proposed method also enhances the above reaction rates leading to complete mineralization of total injected CO₂ volumes within two to twelve months from injection. The invention also discloses various operating conditions such as temperature, pressure, flowrate (depends on rock permeability), etc. that affect the process efficiency, and at which improved sequestration is obtained. Some embodiments of the present invention also cover engineering aspects such as utilizing renewable energy, water looping, and process configuration and design. 

1. A method of carbon dioxide mineralization and storage, comprising: generating CO₂ nanobubbles; injecting said CO₂ nanobubbles as a carbon dioxide and fluid mixture; into a rock formation comprising smafic or ultramafic rock via an injection well wherein the carbon dioxide and fluid mixture is flowed through said injection well or an injection well tubing disposed in the rock formation, the injection well and/or injection well tubing having a plurality of longitudinal perforations at a depth of 0.4 to 4 km in said rock formation; reacting carbon dioxide in said carbon dioxide and fluid mixture with said rock formation to form calcites and magnesites in the rock formation; and recycling at least a portion of the fluid from said rock formation via an observation well.
 2. The method of carbon dioxide mineralization and storage of claim 1, wherein CO₂ nanobubbles are generated by a nanobubble membrane generator.
 3. The method of carbon dioxide mineralization and storage of claim 2, wherein said membrane generator comprises: a pump; an inlet; an air pressure gauge; an air connection; an air flow meter; a pump pressure gauge; a discharge flow valve; and a starter.
 4. The method of carbon dioxide mineralization and storage of claim 3, wherein the CO₂ nanobubbles have an average diameter of about 60-550 nm.
 5. The method of carbon dioxide mineralization and storage of claim 4, wherein the CO₂ nanobubble generator operates at a pressure of at least 25 bar.
 6. The method of carbon dioxide mineralization and storage of claim 5, wherein said fluid mixture comprises at least one selected from the group consisting of water, seawater, brackish water, and conservative tracer.
 7. A method of carbon dioxide mineralization and storage of claim 1, wherein said injection and observation wells have the same depth.
 8. A method of carbon dioxide mineralization and storage of claim 1, further comprising selecting a length and a density of the longitudinal perforations such that the length is at least 15 cm and the density along the injection well tubing and, optionally, a well casing in the injection well based on the fluid flow rate at one or more permeable zones.
 9. A method of carbon dioxide mineralization and storage of claim 5, wherein during the injecting, a high-pressure zone is created within said injection well below a packed off interval and a low-pressure zone is created during the recycling through the observation well.
 10. A method of carbon dioxide mineralization and storage of claim 6, wherein most of the carbon dioxide and fluid mixture flows from a high pressure zone to a low-pressure zone and most of a fluid volume of the carbon dioxide and water mixture is recycled back through the observation well. 